pgsql: Enable archiving in recovery TAP test 009_twophase.pl

Enable archiving in recovery TAP test 009_twophase.pl

This is a follow-up of f663b00, that has been committed to v13 and v14,
tweaking the TAP test for two-phase transactions so as it provides
coverage for the bug that has been fixed.  This change is done in its
own commit for clarity, as v15 and HEAD did not show the problematic
behavior, still missed coverage for it.

While on it, this adds a comment about the dependency of the last
partial segment rename and RecoverPreparedTransactions() at the end of
recovery, as that can be easy to miss.

Author: Michael Paquier
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/743b9b45a2d4013bd90b6a5cba8d6faeb717ee34.camel@cybertec.at
Backpatch-through: 13

Branch
------
REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c360594155e5c0704a952d6b26a01bcbe07cd591

Modified Files
--------------
src/backend/access/transam/xlog.c   | 6 +++++-
src/test/recovery/t/009_twophase.pl | 4 +++-
2 files changed, 8 insertions(+), 2 deletions(-)